1. A computer-implemented control method for optimizing a turndown operation for a power plant having a plurality of generating units for a selected operating period, wherein the plurality of generating units each comprises either an on-condition or an off-condition during the selected operating period, the method including a controller having a processor that performs the steps of: defining competing operating modes, wherein the competing operating modes comprise different possible combinations regarding which of the plurality of generating units comprise the on-condition and which comprise the off-condition during the selected operating period; defining multiple cases for each of the competing operating modes, wherein the multiple cases include varying a value of an operating parameter over a range; receiving performance objectives that include a cost function for evaluating the turndown operation during the selected operating period; receiving an ambient conditions forecast for the selected operating period; for each of the multiple cases of the competing operating modes, simulating the turndown operation of the power plant for the selected operating period with a power plant model given the value of the operating parameter and the ambient conditions forecast; evaluating a simulation result from each of the simulations pursuant to the cost function so to select therefrom a preferred case from the multiple cases for each of the competing operating modes; and controlling the power plant pursuant to the preferred case.