Abstract: Aspects of the invention disclosed herein generally provide heat engine systems and methods for recovering energy, such as by generating electricity from thermal energy. Generally, the heat engine system has a working fluid circuit containing a working fluid (e.g., sc-CO2) for absorbing thermal energy from the heat source stream via a heat exchanger. In one aspect, the method includes controlling a power turbine by modulating a turbo pump throttle valve and a power turbine bypass valve to adjust the flowrate of the working fluid entering the power turbine while monitoring and controlling process operation parameters of the heat engine system to synchronize the frequency of the power generator to the frequency of the electrical grid during a synchronization process.