A method of treating a gas phase fluidized bed reactor and a method of polymerizing olefins in a gas phase fluidized bed reactor in the presence of a catalyst prone to cause sheeting by introducing a chromium-containing compound into the reactor and forming a high molecular weight polymer coating on the walls of the reactor. Furthermore, a device for and method of introducing the chromium-containing compound into the fluidized bed reactor at a plurality of locations in proximity to a lower section of a bed section wall of the fluidized bed reactor, and forming a high molecular weight polymer coating on the bed section wall.