Disclosed is a method and system for determining composition of a fuel entering a combustor. The method includes determining a temperature of the fuel entering the combustor, calculating a first estimated total fuel flow utilizing fuel properties and fuel nozzle effective area (Ae), and calculating a second estimated total fuel flow utilizing an aero-thermal cycle model analysis. The first estimated total fuel flow is compared to the second estimated total fuel flow and a lower heating value of the fuel is determined from a difference between the first estimated total fuel flow and the second estimated total fuel flow. A method and system for controlling a gas turbine includes calculating effects of the fuel composition on performance of the gas turbine and comparing one or more performance parameters to one or more parameter limits. One or more machine controls of the gas turbine are changed based on the results of the comparison.