In a process for quench-cooled, vapor-phase polymerization of olefin monomer comprising (a) contacting an olefin monomer, or mixture of olefin monomers, with a polymerization catalyst in the presence of hydrogen in a reactor vessel to form polymer product, and (b) removing, condensing and recycling off-gas from such reactor, an improvement comprises separating entrained polymerizing solid fines from said off-gas and recycling such solids directly to the reactor without substantial continued polymerization of such solids while in the presence of a substantially different concentration of hydrogen than in the reactor.