Integrated energy system (IES) has the potential to increase the exploitation of renewable energy resources and improve overall energy efficiency. Nevertheless, demand-side uncertainties introduce high risks to the planning of IES, which cannot be resolved by traditional methods. This paper presents a novel real options (RO) based optimal planning approach for integrated electricity, natural gas system and heating systems. First, the random behaviors of load growth are captured through a stochastic module. Thereafter, the optimal planning strategy is obtained through a real option-based planning module and an optimal operation module. Both the total financial value of planning schemes and embedded strategic flexibility value are assessed. The proposed methodology is validated on a large-scale IES consisting of an IEEE 33-bus electricity system, 20-node gas system and 6-node heating system. Results illustrate the effectiveness of the proposed method in planning IES while including long-term uncertainties.