Heaney's widow Marie and one of Lewis's students at Cambridge University, the Rev Roy Taylor, unveiled the works at a ceremony in the Great Hall of Parliament Buildings.
Each bears a famous quotation from the respective writers - the words chosen having particular relevance to the Northern Ireland peace process.
Heaney's quote is "Believe that a further shore is reachable from here" while Lewis's is "There are far better things ahead than any we can leave behind".
Mrs Heaney described the portrait of her late husband as "stunning".
"This is the administrative capital of Northern Ireland, Seamus was a northerner, I am a northerner and I am very glad that he is being honoured here at Stormont, very glad," she said.
"I think it's a wonderful portrait - it shows a great deal of the benign. Seamus was a very benign person and this portrait shows that benign side to him and I am very glad of that."
In regard to the words below Heaney's picture, she added: "It's extremely well chosen and I think everybody should take them on board."
Mr Taylor, from Millisle, Co Down, said it was an honour to unveil the portrait of his famous lecturer.
"He was a very good teacher and very caring for his students," he said.
"He always came in 10 minutes after the o'clock and left 10 minutes before. He said the seats were too hard for his students to sit in for an hour, so he was concerned about our needs. But in those 40 minutes we probably got more than we would have got in two hours with anybody else, he taught so well."
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