the present invention concerns a process and an apparatus for continuous polymerisation of olefin monomers in a cascade of polymerisation reactors . according to the process , an olefin monomer is polymerised first in slurry phase in an inert hydrocarbon diluent in at least one loop reactor and then , subsequently , in gas phase in at least one gas phase reactor . according to the invention , a polymer slurry is continuously withdrawn from the loop reactor and optionally concentrated . the concentrated slurry is conducted to a high pressure flash unit in order to remove the remaining fluid phase , and fed to the gas phase reactor . with the process described in this invention , it is possible to produce bimodal polyethylene with good properties . the operation of the process is stable because of the truly continuous operation .