A process for polymerizing olefin monomer(s) in a gas-solids olefin polymerization reactor comprising 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; comprising the following steps: introducing a fluidization gas stream into the bottom zone; polymerizing olefin monomer(s) in the presence of a polymerization catalyst in a dense phase formed by particles of a polymer of the olefin monomer(s) suspended in an upwards flowing stream of the fluidization gas in the middle zone; introducing a jet gas stream through one or more jet gas feeding ports in a jet gas feeding area of the middle zone at the dense phase in the middle zone of the gas-solids olefin polymerization reactor; wherein the kinetic energy (EJG) input in the reactor by the jet stream is between 1.5 and 50 times higher than the kinetic energy (EFG) input in the reactor by the fluidization gas stream (FG).