A nonlinear mathematical model of a drumtype, twin-furnace, reheat boiler-turbinegenerator system is developed which is suitable for control system analysis and design. The results of an extensive program of comparisons of closed-loop, steady-state operation and open-loop transient responses of field data and model are reported and substantiate the validity of the model. The relatively low order of the model, its high degree of accuracy in representing the relevant responses, and its development on the basis of a "first principle" analysis make it ideal for control system analysis and design. All model parameters are physical quantities which are readily obtainable from design data or unit acceptance test data, and the model is conveniently adapted to other applications.