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: a) introducing a first stream of fluidization gas into the bottom zone; b) 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; c) withdrawing a second stream comprising the fluidization gas from the top zone; d) introducing the second stream into a cooler; e) withdrawing the cooled second stream from the cooler; and f) splitting the cooled second stream into a cooled third stream and the first stream; and g) introducing the cooled third stream through one or more feeding ports in a feeding area of the middle zone at the dense phase in the middle zone of the gas solids olefin polymerization reactor wherein the feeding area of the middle zone is located on the surface of the middle zone between the top end and 50% of the total height of the middle zone, whereas the bottom end corresponds to 0% and the top end corresponds to 100% of the total height of the middle zone. - 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: a) introducing a first stream of fluidization gas into the bottom zone; b) 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; c) withdrawing a second stream comprising the fluidization gas from the top zone; d) introducing the second stream into a cooler; e) withdrawing the cooled second stream from the cooler; and f) splitting the cooled second stream into a cooled third stream and the first stream; and g) introducing the cooled third stream through one or more feeding ports in a feeding area of the middle zone at the dense phase in the middle zone of the gas solids olefin polymerization reactor wherein the feeding area of the middle zone is located on the surface of the middle zone between the top end and 50% of the total height of the middle zone, whereas the bottom end corresponds to 0% and the top end corresponds to 100% of the total height of the middle zone. a) introducing a first stream of fluidization gas into the bottom zone; b) 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; c) withdrawing a second stream comprising the fluidization gas from the top zone; d) introducing the second stream into a cooler; e) withdrawing the cooled second stream from the cooler; and f) splitting the cooled second stream into a cooled third stream and the first stream; and g) introducing the cooled third stream through one or more feeding ports in a feeding area of the middle zone at the dense phase in the middle zone of the gas solids olefin polymerization reactor