The present invention concerns a process for the gas phase polymerization of at least one alpha-olefin comprising: polymerizing at least one alpha-olefin in a fluidized bed reactor with a polyolefin purging unit and a vent gas recovery system, wherein the fluidized bed reactor operates in a condensed mode or super condensed mode with a recycle stream comprising one or more alkane(s) having 3 to 5C atoms, wherein further at least one additional alkane with 6 to 12 C atoms is added to the recycle stream in an amount so that it represents between 0.0095 mol.% and 7,0000 mol.% of the recycle stream composition introduced into the reactor.