A model of grade transition is presented for a commercialized fluidized bed gas-phase polyethyleneproduction process. The quantity of off-specification product and the time of grade transition can be minimizedby the optimization of operating variables, such as polymerization temperature, the ratio of hydrogen to ethylene,the ratio of co-monomer to ethylene, feed rate of catalyst, and bed level. A new performance index, the ratio ofmelt flow (MFR), is included in the objective function, for restraining the sharp adjustment of operation variablesand narrowing the distribution of molecular weight of the resin. It is recommended that catalyst feed rate andbed level are decreased in order to reduce the grade transition time and the quantity of off-specification product.This optimization problem is solved by an algorithm of sequential quadratic programming (SQP) in MATLAB.There is considerable difference between the forward transition and reverse transition of grade with regard to theoperating variables due to the non-linearity of the system. The grade transition model is extended to a high spacetime yield (STY) process with the so-called condensed model operation. In the end, an optimization strategy formulti-product transition is proposed with two-level optimization of the objective function J(x, u) on the basis ofthe optimal grade transition model. A sequential transition of six commercial polyethylene grades is illustrated foran optimal multi-product operation.