The present invention relates to a process and apparatus for producing alpha-olefin polymers in the presence of a polymerization catalyst in a continuously operated multistage polymerisation sequence, the process comprising the steps of (a) polymerizing an alpha-olefin monomer and optionally one or more alpha-olefin co-monomers in a slurry reactor in the presence of a hydrocarbon diluent or liquid monomer in a slurry phase to obtain an alpha-olefin polymer, the slurry phase having a first concentration of solids, (b) continuously withdrawing from the slurry reactor a polymer slurry containing polymer and a fluid phase, (c) concentrating at least a part of the polymer slurry by removing a part of the fluid phase to provide a 1st product stream comprising a concentrated slurry having a second solids concentration, which is higher than the first solids concentration, and a 2nd product stream mainly comprising the fluid phase, (d) polymerizing said 1st product stream in the presence of alpha-olefin monomers and optionally one or more alpha-olefin co-monomers in a first gas phase reactor (GPR1) arranged downstream of said slurry reactor to obtain a 1st alpha-olefin product stream, (e) polymerizing a 3rd product stream, withdrawn from said slurry reactor, in the presence of alpha-olefin monomers and optionally one or more alpha-olefin co-monomers in a second gas phase reactor (GPR2) to obtain a 2nd alpha-olefin product stream.