The present disclosure provides a method of maintaining a target value of a melt flow index of a polyethylene polymer product being synthesized with a metallocene catalyst in a fluidized bed gas phase reactor. The method includes producing the polyethylene polymer product at the target value of the melt flow index with a metallocene catalyst in a fluidized bed gas phase reactor at a steady state in which the fluidized bed gas phase reactor is at a first reactor temperature and receives feeds of hydrogen and ethylene at a hydrogen to ethylene feed ratio at a first ratio value. When a change in reactor temperature is detected, the hydrogen to ethylene feed ratio is changed from the first ratio value to a second ratio value so as to maintain the melt flow index value of the polyethylene polymer product at the target value.