The union said it would be appealing the decision and would take Ineos to an employment tribunal.
Pat Rafferty, Unite's Scottish secretary, said the convener, Mark Lyon, had been "subjected to a grotesque mockery of the disciplinary system which saw him tried in his absence".
He added: "While he attended a doctor's appointment, his employer was sacking him on trumped-up charges. From start to finish this has been a circus and reflects extremely poorly on Ineos."
Mr Rafferty also accused the company of victimising the previous convener Stevie Deans, the man at the centre of a row last year which saw Ineos threaten to close part of the site if workers did not agree to a new package.
Ineos refused to comment, saying it was a "confidential issue between the company and Mr Lyon".
The row came as Labour finalised plans to reshape its relationship with the unions in the wake of another related row over Falkirk. The changes follow allegations of vote-rigging in Falkirk, when Mr Deans was also the chairman of the local Labour party.
The Unite union was accused of packing the constituency party to ensure its favoured choice became Labour's candidate for the 2015 General Election. There were also claims members were signed up to Labour without their knowledge. The union was cleared after a party investigation.
However, it is understood that the party's controversial gift membership scheme will remain in place, allowing people to be signed up to Labour if a membership is bought for them.
Under the plans, Labour would change how its leaders are chosen and how union members sign up to the party. But the reforms will not be fully implemented for another five years.
The proposals were backed by Labour's ruling National Executive Committee yesterday and will now go to a special party conference on March 1. If approved, they will force union members to choose whether or not to opt into paying affiliation fees to Labour.
Union members will also be given an opportunity to become affiliated Labour members, allowing them to vote in leadership elections.
Labour will also scrap the "electoral college" system for leadership votes, bringing in one-member one-vote, while London members will also be offered a primary election to choose the party's candidate for mayor.
Mr Miliband suggested the reforms would make the party in "touch with working people and people from all walks of life". He said: "These changes will help bridge the gap between Westminster and the rest of Britain.
"They are about opening up the Labour Party so that more people from every walk of life can have more say on the issues which matter to them most like the cost-of-living crisis."
But MPs from his party have warned the plans could mean the party loses millions of pounds in union affiliation fees.
There have also been warnings the move, which will cut the power of MPs to choose their leader, could give unions more influence over those at the top of the party.
Party conferences, where affiliated organisations including unions have 50% of the vote, will remain unchanged for at least five years. Labour sources admit the changes will hurt the party financially.
Some estimates put the total potential loss at around £4 million.
A party source said: "We want to get to a place where we grow the number of our small donors who give small amounts of money. They are already our biggest single source of income.
"We want to grow that base even more."