The present invention relates to a continuous gas phase process comprising polymerizing one or more hydrocarbon monomer(s) in a fluidized bed reactor in the presence of a Ziegler-Natta-type catalyst system and a condensable fluid for a period of at least 12 hours where the bed temperature is less than the Critical Temperature and the dew point temperature of the gas composition in the reactor is within 25.degree. C. of the bed temperature.