Process for polymerizing monomer in a reactor by withdrawing an effluent from the reactor which is a slurry of polymer solids in a liquid medium and contacting a first stream containing vapor derived from the effluent withdrawn from the reactor with a feed stream to the reactor. The first stream which has the majority of the vaporized components of the liquid medium in the effluent withdrawn from the reactor is brought into contact with the feedstream to the reactor at a pressure of 0.5 MPa or above and/or the first stream is separated from the polymer withdrawn from the reactor at a pressure of 0.5 MPa or above.