Process for introducing a solid polymerization catalyst into a gas-phase fluidized bed using an injection device having an inner tube of internal cross-sectional area of 10 to 100 mm2 and an outer tube forming an annulus around the inner tube with a cross-sectional area of 1 to 10 times the internal cross-sectional area of the inner tube. The polymerization catalyst and a carrier gas are passed through the inner tube and into the gas-phase fluidized bed at a carrier gas linear velocity of 4 to 14 m/s and a carrier gas mass flow rate of 10-35 kg/h. A shielding gas is passed through the outer tube and into the gas-phase fluidized bed at a linear velocity of 1 to 10 times the carrier gas linear velocity through the inner tube and at a mass flow rate of 100-500 kg/h. No cooled recycle process gas is provided to the injection device.