A method of transitioning from a first catalyst to a second catalyst in a gas phase fluidized bed reactor comprising continuously feeding the first catalyst and a recycle stream comprising olefin monomer to the reactor; wherein the monomer contacts the first catalyst in the fluidized bed and polymerizes; wherein the reactor is operating in condensing mode (withdrawing a gaseous stream comprising unreacted monomer from the reactor, cooling the gaseous stream to condense a portion thereof, and contacting the cooled gaseous stream with fresh monomer to form the recycle stream); and wherein a liquid phase of the recycle stream evaporates within the fluidized bed; discontinuing the first catalyst to the reactor while continuing to feed the recycle stream; maintaining the condensing mode in reactor at >3 wt. % liquid phase in recycle stream while no fresh catalyst is introduced to reactor; and introducing the second catalyst to the reactor operating in condensing mode.