Disclosed herein is a gas phase polymerization process including the steps of passing a recycle stream through a fluidized bed in a gas phase fluidized bed reactor in the presence of a bulk density promoting agent, wherein the bulk density promoting agent is of a saturated hydrocarbon and/or a non-polymerizable unsaturated hydrocarbon having 6 or more carbon atoms; polymerizing at least one alpha-olefin monomer in the presence of a catalyst to produce an alpha-olefin polymer; and controlling an amount of the bulk density promoting agent in the reactor such that a bulk density of the alpha-olefin polymer discharged from the reactor is greater than or equal to about 480 kg/m3.