A hybrid atomistic-continuum method devoted to the study of multi-scale problems is presented. The simulation domain is decomposed into three regions: the bulk where the continuous Navier-Stokes and energy equations are solved, the neighborhood of the wall simulated by the Molecular Dynamics and the overlap region which connects the macroscopic variables (velocity and temperature) between the two former regions. For the simulation of long micro/nanochannels, we adopt multiple molecular blocks along the flow direction to capture accurately the velocity and temperature variations from the inlet to the outlet. The validity of the hybrid method is shown by comparisons with a simplified analytical model in the molecular region. The method is then applied successfully to the study the hydrodynamic and thermal development of a liquid flow in a long micro/nanochannel without and with condensation.