Using Molecular Dynamics (MD) Simulations and Theoretical Calculations, We Study Heat Transfer Across Liquid-Gas Interfaces within a Planar Heat Pipe. to Determine the Thermal Conductance (Kapitza Conductance), GK, at the Interface, Two Heat Transfer Mechanisms, Namely, Conduction and Evaporation/condensation Are Considered. in the Case of Interfacial Heat Conduction, Gas Molecules, Particularly Non-Condensable Gas Molecules, Exchange Heat with Liquid Surfaces through Gas-Liquid Collisions, and the Theoretical Expression for GK is Derived from the Kinetic Theory of Gases. for Interfacial Heat Transfer by Evaporation or Condensation, the Theoretical Expression for GK is Derived from the Schrage Relationships. to Assess the Accuracies of the Theoretical Expressions for GK, We Compare These Theoretical Predictions to the GK Obtained Directly from MD Simulations. for All Cases Studied, the Theoretical Predictions Agree with the MD Simulation Results Very Well. If the Density of Non-Condensable Gas in the Heat Pipe is Much Higher Than that of the Working Fluid in the Gas Phase, We Find that the Interfacial Heat Conduction Could Contribute Significantly to the Total Heat Flux Across the Liquid-Gas Interfaces. the Effect of GK at Liquid-Gas Interfaces on the overall Heat Transfer Efficiency in a Planar Heat Pipe is Discussed.