The primary control objective for a certain industrial polymer reactor is to control the polymer product melt viscosity; and two input variables, the reactor temperature, and the mole fraction of the chain transfer agent in the reactor, are available for this purpose. However, because the latter affects the melt viscosity on a much slower timescale than the former, reactor temperature is used as the primary control variable when close to steady-state operating conditions. An approximate model for this aspect of the process under such conditions is given by