The decarbonisation of urban energy systems requires integrated planning approaches capable of coordinating different energy vectors, conversion technologies, and storage systems within a common framework. This thesis develops a mixed-integer linear programming model for the integration of a district heating network into a district-scale multi-energy system. The proposed framework couples the thermal network with the electrical system and jointly optimises the sizing and hourly operation of a wide portfolio of technologies, including photovoltaic and solar thermal systems, boilers, heat pumps, combined heat and power units, battery storage, and thermal energy storage. In this way, thermal demand, electricity consumption, energy conversion, and storage management are treated as interdependent elements of the same multi-vector energy system.