A method of transitioning a gas-phase polyolefin reactor system, the gas-phase polyolefin reactor system including a gas-phase polyolefin reactor, from production of a first polyolefin product to production of a second polyolefin product, wherein during the transition the flow rate of one or more reaction components is changed, comprising the steps of: a) providing the gas-phase polyolefin reactor system comprising a steady-state control valve and a high-flow valve, wherein a first, input gas stream comprising a reaction component is supplied from a header to the gas phase polyolefin reactor through the steady-state control valve or through the high flow valve, and in particular is connected to an input end of the steady state control valve and to an input end of the high-flow valve and a second, output gas stream is connected to an output end of the steady-state control valve and to an output end of the high-flow valve, wherein the second, output gas stream is connected to a third, recycle gas stream, wherein the third, recycle gas stream is connected to an input of the gas-phase polyolefin reactor; wherein the first, second and third streams comprise the reaction component and wherein the reaction component is selected from the group consisting of hydrogen, monomers, co-monomers, co-catalysts and catalyst modifiers, b) supplying the input gas stream containing the reaction component to the gas-phase polyolefin reactor to produce the first polyolefin product, wherein the input gas stream is directed through the steady-state control valve and wherein the input gas stream enters via the second, output gas stream into the third, recycle gas stream and then the reactor; c) selecting an ending reaction component concentration in the polyolefin reactor required for the second polyolefin product; d) predicting, based upon a measured starting reaction component concentration and the selected ending reaction component concentration, a required amount of reaction component for the transition using an empirical model between the amount of reaction component and the concentration of reaction component; e) directing the input gas stream comprising the reaction component through the high-flow valve, to the second, output gas stream which enters the third, recycle gas stream and the third recycle gas stream carries the reaction component into the gas phase polyolefin reactor, to thereby commence transitioning the gas-phase polyolefin reactor system from production of the first polyolefin product to production of the second polyolefin product; f) measuring a flow rate of the first input gas stream near the input end of the high-flow valve with a flow meter; g) estimating a transition endpoint using the empirical model and a totalizer based upon the required amount of reaction component and the measured flow rate of the first, input gas stream; and h) automatically closing the high-flow valve at the transition endpoint. a) providing the gas-phase polyolefin reactor system comprising a steady-state control valve and a high-flow valve, wherein a first, input gas stream comprising a reaction component is supplied from a header to the gas phase polyolefin reactor through the steady-state control valve or through the high flow valve, and in particular is connected to an input end of the steady state control valve and to an input end of the high-flow valve and a second, output gas stream is connected to an output end of the steady-state control valve and to an output end of the high-flow valve, wherein the second, output gas stream is connected to a third, recycle gas stream, wherein the third, recycle gas stream is connected to an input of the gas-phase polyolefin reactor; wherein the first, second and third streams comprise the reaction component and wherein the reaction component is selected from the group consisting of hydrogen, monomers, co-monomers, co-catalysts and catalyst modifiers, b) supplying the input gas stream containing the reaction component to the gas-phase polyolefin reactor to produce the first polyolefin product, wherein the input gas stream is directed through the steady-state control valve and wherein the input gas stream enters via the second, output gas stream into the third, recycle gas stream and then the reactor; c) selecting an ending reaction component concentration in the polyolefin reactor required for the second polyolefin product; d) predicting, based upon a measured starting reaction component concentration and the selected ending reaction component concentration, a required amount of reaction component for the transition using an empirical model between the amount of reaction component and the concentration of reaction component; e) directing the input gas stream comprising the reaction component through the high-flow valve, to the second, output gas stream which enters the third, recycle gas stream and the third recycle gas stream carries the reaction component into the gas phase polyolefin reactor, to thereby commence transitioning the gas-phase polyolefin reactor system from production of the first polyolefin product to production of the second polyolefin product; f) measuring a flow rate of the first input gas stream near the input end of the high-flow valve with a flow meter; g) estimating a transition endpoint using the empirical model and a totalizer based upon the required amount of reaction component and the measured flow rate of the first, input gas stream; and h) automatically closing the high-flow valve at the transition endpoint.