Quoting the tribunal judges, chief prosecutor Tuhin Afroz said the court thought Alim deserved the death sentence for his crimes, but handed him a lesser penalty considering his old age and illness.
Alim's party did not give any immediate reaction to the verdict, but it has called the trial politically motivated.
Last week the tribunal sentenced another senior party member, Salahuddin Quader Chowdhury, to death for crimes against humanity.
The verdict came as the opposition party, led by Hasina's main rival Khaleda Zia, is campaigning for her resignation and transfer power to a neutral caretaker administration to make the next general election free and fair. The election is due in January.
Some international rights groups have echoed the opposition's allegation that the trials are flawed and don't meet international standards, which the government denies.