Installation for the gas-phase polymerisation of at least one olefinic monomer, comprising a horizontal stirred reactor consisting of an undivided space, provided with a number of gas feeds (13a-13) in the bottom section of the reactor (1) and a number of liquid feeds (7a-7) in the top section of the reactor (1) and at least two gas outlets (9, 11) at the top of the reactor (1), the installation being provided with means (43, 45) to regulate the discharge capacities of the gas outlets.