1. An adaptive process controller for controlling one or more devices within a process plant, the adaptive process controller comprising: a controller having a prediction unit that uses a prediction model to determine a prediction signal for a set of one or more process variables, a prediction error unit that combines the prediction signal with a set point signal to produce a prediction error signal, and a combiner that multiplies the prediction error signal by a controller gain signal to produce a change in a control signal and that uses the change in the control signal to develop a control signal for controlling one of the set of process variables; a process model estimation unit communicatively coupled to one or more devices within the process plant to collect process data during the on-line operation of the process plant and that determines a new process model representing the operation of a portion of the process plant from the collected process data, the new process model including a set of model parameters defining characteristics of the portion of the process plant; and a controller adaptation unit that uses a closed form equation that expresses a new controller gain signal as a function of at least one of the set of process model parameters of the new process model to calculate the new controller gain signal and that adapts the controller to use the new controller gain signal while the controller is operating on-line to control the portion of the process plant.