forming an ethylene polymer in particle form in a gas-phase polymerization unit comprising a gas-phase polymerization reactor by homopolymerizing ethylene or copolymerizing ethylene and one or more C₄-C₁₂-1-alkenes at temperatures from 20 to 200° C. and pressures of from 0.5 to 10 MPa in a reaction gas comprising propane as polymerization diluent in the presence of a polymerization catalyst,

wherein a purified propane feed stream comprising at least 99% mol propane and not more than 100 ppm-mol propylene is fed into the gas-phase polymerization unit and purification of the purified propane feed stream has been carried out by passing a hydrocarbon stream containing propane, propylene, acetylene, carbon monoxide, carbon dioxide, oxygen and water through hydrocarbon purification units which reduce at least a concentration of propylene, acetylene, carbon monoxide, carbon dioxide, oxygen and water contained in the hydrocarbon stream to produce the purified propane feed stream.