10-Gigabit-capable Passive Optical Network (XG-PON), one of the latest standards of optical access networks, is regarded as one of the key technologies for future Internet access networks. In this paper, we propose and discuss the design and implementation of an XG-PON module for the NS-3 network simulator. The aim is to provide a standards-compliant, configurable, and extensible module that can simulate XG-PON with reasonable speed and can support a wide range of research topics. These include analysing and improving the performance of XG-PON, studying the interactions between XG-PON and the upper-layer protocols, and investigating its integration with various wireless networks.