A natural gas containing 95 mole% methane and the balance of ethane is burned with 25% excess air. The stack gas leaves the furnace at 900 degrees C and 1 atm and contains no unburned hydrocarbons or carbon monoxide. The gas is cooled to 500 degrees C in a heat exchanger. Taking as a basis of calculation 100 mol of the fuel gas fed to the furnace, calculate the amount of heat (kJ) that must be transferred from the gas in the heat exchanger to accomplish the indicated cooling.