Method of controlling a polymerization reactor

A method of controlling a control variable of a polymerization reactor independently of controlling other polymerization conditions in a gas-phase polymerization process, comprising:

• (a) continuously, periodically or intermittently determining a difference between an actual or estimated value of the control variable and a desired value of the control variable;
• (b) adjusting or maintaining at least a first manipulated variable to at least partially compensate for the difference between the actual or estimated value of the control variable and the desired value;
• and
• (c) adjusting or maintaining a second manipulated variable to at least partially compensate for the effect of adjusting or maintaining the first manipulated variable;
• wherein the first and second manipulated variables are selected from the group of process conditions consisting of a weight of fluidized bed, a concentration of one or more monomers, a flow of one or more comonomers, a ratio of a first comonomer to a second comonomer, an first activator concentration, a ratio of a second activator to selectivity control agent, a concentration of a chain transfer agent, and a retardant concentration.