Recent advances in energy harvesting (EH) technologies now allow wireless sensor networks (WSNs) to extend their lifetime by scavenging the energy available in their environment. While simulation is the most widely used method to design and evaluate network protocols for WSNs, existing network simulators are not adapted to the simulation of EH-WSNs and most of them provide only a simple linear battery model. Therefore, there is a need for a framework suited to EH-WSN simulation and to lifetime prediction. We propose a co-simulation framework, HarvWSNet, based on WSNet and Matlab, that provides adequate tools for the simulation of the network protocols and the lifetime of EH-WSN. Indeed, the framework allows for the simulation of multi-node network scenarios while including a detailed description of each node's energy harvesting, management subsystem and its time-varying environmental parameters. A simulation case study based on a temperature monitoring application demonstrates HarvWSNet's ability to predict network lifetime while minimally penalizing simulation time.