Power-to-X is a promising technology for decarbonising parts of the industry and the transport sector. The former Danish government formulated a national strategy for Power-to-X to reach a capacity of 4-6GW of electrolysis capacity by 2030. The capacity would result in a large waste heat stream due to heat loss. Since the existing research on the utilisation of waste heat is limited, this thesis investigated how the integration of waste heat from Power-to-X can be utilised in the district heating systems of Sønderborg. Waste heat from Power-to-X differs from the dominant technologies since it is a fluctuating heat source that cannot be controlled directly by the district heating companies and the utilisation may require a stronger cooperation of the district heating companies in the area. The quantitative analyses showed that there is a potential for expanding the district heating system and the total expansions could increase the annual heat demand by 15%. The utilisation of the waste heat depends on whether the system is interconnected or two separate systems. Compared to the existing system, the utilisation of the waste heat is feasible from both a business and socioeconomic perspective if the two district heating systems are connected by a transmission line. Depending on the size of the Power-to-X plant, waste heat could contribute 34-56% of the annual heat production if the systems are interconnected.