Abstract: A process for the production of polyolefins comprising: feeding a slurry comprising at least one polymerization catalyst, at least one carrier liquid, first olefin monomer(s) and optionally at least one first comonomer into at least one loop reactor; polymerizing the first olefin monomer(s) and optionally the at least one first comonomer yielding a first polyolefin; withdrawing the first polyolefin from the loop reactor; feeding the first polyolefin to a gas-solids olefin polymerization reactor, wherein the gas-solids olefin polymerization reactor comprises: a top zone; a middle zone, which comprises a top end in direct contact with said top zone and which is located below said top zone, the middle zone having a generally cylindrical shape; and a bottom zone, which is in direct contact with a bottom end of the middle zone and which is located below the middle zone; introducing a fluidization gas stream into the bottom zone of the gas-solids olefin polymerization reactor; polymerizing second olefin monomer(s)