Abstract: This invention relates to a novel horizontal fluid bed reactor, which in a single unit, provides for the polymerization of monomer to polymer and mixed monomers to co-polymers. Within one reactor shell the reactor contains a plurality of polymerization stages which permit the achievement of narrow residence time distribution of the produced polymer. This arrangement also creates a sufficiently tight subdivision of the reactor volume which permits a plurality of gas circulation stages of different gas compositions, as well as, polymerization temperatures.