An improved process for the polymerization of olefin monomer selected from (a) ethylene, (b) propylene, (c) mixtures of ethylene and propylene and (d) mixtures of (a), (b) or (c) with one or more alpha-olefins in a fluidized bed gas phase reactor, said process comprising passing a gaseous mixture of said olefin monomers(s) through the fluidized bed under effective polymerization conditions to provide a polymer product containing unreacted monomer(s) and a gaseous effluent stream comprising unreacted monomer(s), removing the polymer product to a degassing vessel, recycling a first portion of the gaseous effluent stream to the fluidized bed and passing a second portion of the gaseous effluent stream to counter currently contact the polymer product in said degassing vessel to produce a polymer product having a reduced amount of unreacted monomer(s), characterized in that the improvements to the process comprise: (i) removing heavy hydrocarbons from the second portion of the effluent stream and returning said hydrocarbons to the first portion of the effluent stream for recycle to the fluidized bed, (ii) removing substantially all of the monomer(s) from the second portion of the effluent stream and returning said monomers(s) to the first portion of the effluent stream for recycle to the fluidized bed, and (iii) recycling the second effluent stream after passage through the degassing vessel to the gaseous effluent stream.