A control method for optimizing a turndown operation for a power plant having a plurality of generating units. The plurality of generating units each may include either an on-condition or an off-condition during a selected operating period. The method may include: defining competing operating modes, wherein the competing operating modes include different possible combinations regarding which of the plurality of generating units have the on-condition and which have 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; receiving performance objectives that include a cost function; receiving an ambient conditions forecast; for the multiple cases of the competing operating modes, simulating the turndown operation with a power plant model given the value of the operating parameter and the ambient conditions forecast; evaluating a simulation result pursuant to the cost function so to select therefrom a preferred case.