Embodiments of the invention generally provide a heat engine system, a method for generating electricity, and an algorithm for controlling the heat engine system which are configured to efficiently transform thermal energy of a waste heat stream into electricity. In one embodiment, the heat engine system utilizes a working fluid (e.g., sc-CO2) within a working fluid circuit for absorbing the thermal energy that is transformed to mechanical energy by a turbine and electrical energy by a generator. The heat engine system further contains a control system operatively connected to the working fluid circuit and enabled to monitor and control parameters of the heat engine system by manipulating a power turbine throttle valve to adjust the flow of the working fluid. A control algorithm containing multiple system controllers may be utilized by the control system to adjust the power turbine throttle valve while maximizing efficiency of the heat engine system.