the present invention concerns a process and an apparatus for continuous polymerisation of olefin monomers . in particular , the present invention concerns a process and an apparatus for continuous polymerisation olefin monomers like ethylene and other monomers , wherein an olefin monomer is polymerised in slurry phase in an inert hydrocarbon diluent in at least one loop reactor . according to the invention , a polymer slurry is continuously withdrawn from the loop reactor and concentrated . the concentrated slurry is conducted to a flash unit in order to remove the remaining fluid phase , and gas obtained is compressed in a flash gas compressor before it is being fed to a distillation section . by means of the present invention it is possible reduce the size of the flash gas compressor and the capacity of the distillation sections . this significantly reduces investment cost for a continuously operated polymerization apparatus .