in polymerizing an olefin , for example , in a polymerization reactor with a reflux condenser , a gas mixture withdrawn from the polymerization reactor is first washed before it reaches the heat transfer surface of the reflux condenser to remove active catalyst particles and polymer particles entrained therein and then condensed to obtain a condensed liquid , and the condensed liquid is returned to the polymerization reactor whereby the heat generated in the polymerization reactor is efficiently removed without causing any clogging of pipes , etc .