A process for polymerizing one or more olefins in a gas-phase polymerization reactor is provided. The gas-phase reactor has a fluidized bed and a fluidizing medium. The fluidizing medium has an operating density and an operating velocity. The process includes determining a critical gas velocity and/or determining a critical gas velocity for the polymerization. The operating gas density and/or the operating gas velocity for the fluidizing medium is then adjusted to be less than or equal to its respective critical value. The process includes increasing the bulk density of the fluidized bed. The increase in the fluidized bed bulk density increases productivity without increasing reactor residence time.