A process for the polymerization of olefins in gas phase carried out in a reactor having two interconnected polymerization zones, a first zone (riser) and a second zone (downcomer), wherein growing polymer particles: a) flow through the riser under fast fluidization conditions established by feeding a mixture of gas and liquid; b) leave the riser and enter the downcomer, through which the growing polymer particles flow downward in a densified form; and c) leave the downcomer and are reintroduced into the riser, thereby establishing a circulation of polymer between the riser and the downcomer; the reactor is operated at a temperature between 0° C. and 5° C. above the dew point of the riser gas at the operating pressure, and in the riser, besides the growing polymer particles and gas flow, an amount of liquid is present.