1. A method for simultaneous synthesis of combined heat and power utility and process subsystems for a facility or cluster of process facilities, the method comprising the steps of: defining a plurality of rigorously bounded process variables for one or more process facilities as decision variables to thereby form model input data for a combined heat and power utility and process subsystem mathematical program model, the decision variables comprising a process steam demand interval at each of a plurality of different steam pressure levels and a process power generation interval describing the respective upper and lower bounds; processing, by a computer, the model input data by the combined heat and power utility and process subsystem mathematical program model; and determining, by the computer, a plurality of at least substantially optimal process and utility subsystem conditions, the process conditions comprising a process steam demand load value at each of the plurality of different steam pressure levels defining model output.