In a process for the polymerization of C2-C8-alk-1-enes by means of a Ziegler-Natta catalyst system, in which polymerization is carried out from the gas phase in at least one reaction zone at from 40 to 120° C. and pressures of from 1 to 100 bar, the pressure and temperature in the reaction zone are set so that an operating point formed by these parameters in a pressure-temperature diagram is located at from 0.2 to 5.0 bar below the dew line of the respective reaction mixture above which condensation of the C2-C8-alk-1-enes occurs.