Remember that the heat of combustion (Also known as the reaction enthalpy in this case) is the difference between the total enthalpy of the formation of the products and the total enthalpy of formation of the reactants. Remember that when dealing with thermodynamic situations, it doesn't matter how you get there just as long as you get there. So when calculating the enthalpy of a reaction, the enthalpy of formation can be used. (As this makes the assumption that all the reactants break down into their elements, and all the products are formed from their elements). Hence the enthalpy of formation of a pure element is zero since all the values are determined based upon formation from pure elements. A pure element is a pure element, so it has no heat of formation.
So with your products, you are trying to determine how much energy is given off when the products form directly from their elements. With the reactant, you have to determine how much energy is needed to break it down into the elements it's made of. When you get the difference between these two values, you'll have your heat of combustion of one mole of propane.