The introduction of hydrogen and natural gas blends in existing gas transportation and distribution networks would ensure faster and more efficient decarbonization of energy sector, but, at the same time, this process would request solution of many practical and technical problems. This paper explores thermodynamics of hydrogen and natural gas blend combustion products and estimates the amount of condensate and latent energy recovery from flue gas as a function of condensing temperature. The efficient energy recovery epends on network return temperature, and it is possible to overcome this limitation by implementation of heat pump for extraction of low temperature heat from flue gases. The case study considers operation of heat only boiler and flue gas condenser with integrated cascade of heat pumps, which consist of absorption lithium bromide-water chiller (in heat pump mode) and vapour compression unit. Presented results of energy recovery hence are limited by data collected from the natural gas combustion for district heating network energy supply. However, previous thermodynamic consideration allows extending the obtained results for case of hydrogen and natural gas blend combustion. A proof of concept of heat recovery by combination of flue gas condenser supported by a cascade of heat pumps demonstrates the efficiency in case of hydrogen and natural gas blend combustion.