In this paper various methods for calculation of gross and net heat of combustion for different biopolymers (lignin, cellulose, hemicelluloses, starch, pectin, proteins, lipids etc.) have been analyzed. The results showed that the calculation with use the energy released by combustion per gram of diatomic oxygen (Eq-parameter) is less accurate, because it gives a deviation from experimental values of more than 6%. In the case of calculations based on contribution of structural groups of polymers, the deviation for some samples may reach 6%. The lowest deviation in the range ±1% was obtained using an improved method of calculation, which is based on elemental composition of the polymers. Calculation of gross and net heat of combustion for biomass samples by the improved method was very close to experimental calorific values. It was found that combustion of plant biomass waste supplemented with waste plastic is preferable, since such combustion technology provides more thermal energy than single firing of biomass and is accompanied by less emission of carbon dioxide in comparison with separate burning of plastic waste only.