1. A controller for controlling a multi-variable input plant having a plurality of manipulable variables (Mvs) as inputs, and operable to provide a plurality of measurable outputs and at least one unmeasurable output, which unmeasurable output can not be measured in substantially real time and requires periodic external analysis for the determination of periodic values of the at least one unmeasurable output, comprising: a plant predictive model that provides a model of the dynamics of selected aspects of the operation of the plant for modeling the dynamics thereof and providing at least one predicted output for at least a select one of the measurable outputs; an external predictive model, wherein, responsive to at least one input that is not an MV, the external predictive model is operable to: predict intermediate values of the at least one unmeasurable output in substantially real-time for use between use of the periodic values of the at least one unmeasurable output, and a desired unmeasurable output value for that at least one unmeasurable output; and predict the dynamics of a select one of the measurable outputs as a function of the at least one unmeasurable output and the desired value for the at least one unmeasurable output to predict the dynamics as a desired value of the select one of the measurable outputs required to achieve the desired unmeasurable output value; an optimizer for receiving desired values for the selected aspects of the operation of the plant modeled by said predictive model and said predicted outputs from said predictive model in addition to the desired value of the select one of the measurable outputs generated by said external predictive model and optimizing the inputs to the predictive model to minimize error between the predicted and desired values; and a control input device for applying the optimized input values to the plant after optimization thereof.