IEEE 802.15.4g defines smart metering utility networks (SUNs) to enable system control and information transfer in smart grid. However, sharing operation on unlicensed bands makes most SUN channels to overlap with wireless local area network (WLAN) or ZigBee channels in a heterogeneous communication environment. Thus, coexistence of SUNs and WLANs/ZigBees is a crucial issue, which is the focus of this paper. In particular, we will find a solution to mitigate WLAN and ZigBee interferences on SUNs operating in the same band. Analytical model is applied to evaluate bit error rate (BER) performance of a SUN in different interference scenarios. The simulation shows that frequency offset and separation distance play important roles in coexistence situations. We then use a packet error rate (PER) calculation model to analyze the PER performance in order to determine the minimum separation distances between a SUN receiver and WLAN/ZigBee transmitters. Finally, a coexistence solution based on multiple gateway wireless mesh topology is proposed for SUNs to mitigate WLAN/ZigBee interferences.