A gas-phase polyolefin reactor system for rapidly transitioning from one polyolefin product to another is disclosed. The reactor system comprises a steady-state control valve, a high-flow valve, a polyolefin reactor, a flow meter, a totalizer, an online analyzer and an empirical model. During a transition, the on-line analyzer measures an initial concentration of a reaction component, the empirical model predicts a required amount based upon the measured initial concentration and a selected ending concentration, the flow meter measures a flow rate, the totalizer determines a totalized amount when the flow rate of the first stream reaches the required amount based upon the measured flow rate and outputs the totalized amount to the empirical model, and the empirical model compares the required amount to the totalized amount and determines a transition endpoint. A method of rapidly transitioning the reactor system from one polyolefin product to another is also disclosed.