He was 88.
Heap, who had suffered from Alzheimer's in recent years, died Friday at a Toronto nursing home.
His death was announced on Facebook by his son, David, who described his father as a "pacifist, socialist, worker-priest, Marxist Anglican, trade-unionist, city councillor, member of parliament, civilly disobedient marcher for human rights."
Heap was a Toronto MP for 12 years before stepping down from federal politics in 1993.
Former NDP MP Olivia Chow, who once worked for Heap and calls him a mentor, says Heap was "never a person about himself."
"He was never seeking power or glory. He was always standing up for his principles of justice and peace," she said in an interview on Saturday.
"And he's done a tremendous amount championing anti-poverty, homelessness and human rights issues his entire life."