Process for heating a polymer-containing stream being transferred from a polymerization reactor to a degassing vessel operating at a pressure between 6 bara and 12 bara. The process includes passing the stream through a heater having a transfer line for the stream and a device for heating the transfer line. The pressure drop in the heater is between 5% and 50% of the total pressure drop between the polymerization reactor and the entry to the degassing vessel. The pressure drop across the length of the heater is less than 0.5 barh per tonne of polymer, and the average Reynolds number across the cross-section of the stream at any point along the length of the transfer line of the heater is greater than 500,000, such that at least 90 mol% of the hydrocarbon fluids withdrawn from the polymerization reactor operation are vaporized before entry into the degassing vessel.