The invention relates to a multi-zone reactor for the continuous fluidized bed polymerization of one or more α-olefin monomers of which at least one is ethylene or propylene, which multi-zone reactor is operable in condensed mode, which multi-zone reactor comprises a first zone, a second zone, a third zone, a fourth zone and a distribution plate, wherein the second zone contains an inner wall, wherein the third zone contains an inner wall, wherein at least part of the inner wall of the third zone is either in the form of a gradually increasing inner diameter or a continuously opening cone, wherein the diameter or the opening increases in the vertical direction towards the top of the multi-zone reactor, wherein the largest diameter of the inner wall of the third zone is larger than the largest diameter of the inner wall of the second zone.