A process for preparing an olefin polymer, comprising the steps of a) forming a particulate olefin polymer in a gas-phase polymerization reactor by homopolymerizing an olefin or copolymerizing an olefin and one or more other olefins at temperatures from 20 to 200°C and pressures of from 0.5 to 10 MPa in the presence of a polymerization catalyst and a reaction gas comprising a C 3 -C 5 alkane as polymerization diluent; b) discharging the formed polyolefin particles from the polymerization reactor into a separation vessel being operated at a pressure from 1 to 2.2 MPa; c) withdrawing a gaseous stream from the separation vessel and transferring the gaseous stream into a reaction gas work-up unit, which is operated at a pressure from 0.001 to 0.2 MPa below the pressure of the separation vessel and which produces at least one liquid stream which is recycled to the polymerization reactor; d) transferring the polyolefin particles from the separation vessel into a first degassing vessel operated at a pressure from 0.1 to 0.4 MPa; and e) contacting therein the polyolefin particles with a gaseous stream comprising from 85 to 100 vol.-% of a C 3 -C 5 alkane, wherein the gaseous stream is continuously received from the reaction gas work-up unit; f) withdrawing the gaseous stream, which had been in contact with the polyolefin particles in the first degassing vessel, from the first degassing vessel, compressing the gaseous stream to the pressure of the gaseous stream withdrawn from the separation vessel, and transferring the compressed gaseous stream into the reaction gas work-up unit, in which the gaseous stream withdrawn from the separation vessel is worked up. a) forming a particulate olefin polymer in a gas-phase polymerization reactor by homopolymerizing an olefin or copolymerizing an olefin and one or more other olefins at temperatures from 20 to 200°C and pressures of from 0.5 to 10 MPa in the presence of a polymerization catalyst and a reaction gas comprising a C 3 -C 5 alkane as polymerization diluent; b) discharging the formed polyolefin particles from the polymerization reactor into a separation vessel being operated at a pressure from 1 to 2.2 MPa; c) withdrawing a gaseous stream from the separation vessel and transferring the gaseous stream into a reaction gas work-up unit, which is operated at a pressure from 0.001 to 0.2 MPa below the pressure of the separation vessel and which produces at least one liquid stream which is recycled to the polymerization reactor; d) transferring the polyolefin particles from the separation vessel into a first degassing vessel operated at a pressure from 0.1 to 0.4 MPa; and e) contacting therein the polyolefin particles with a gaseous stream comprising from 85 to 100 vol.-% of a C 3 -C 5 alkane, wherein the gaseous stream is continuously received from the reaction gas work-up unit; f) withdrawing the gaseous stream, which had been in contact with the polyolefin particles in the first degassing vessel, from the first degassing vessel, compressing the gaseous stream to the pressure of the gaseous stream withdrawn from the separation vessel, and transferring the compressed gaseous stream into the reaction gas work-up unit, in which the gaseous stream withdrawn from the separation vessel is worked up.