A unique heat rejection system for space applications is proposed herein by integrating a variable conductance planar heat pipe with a radiator panel. The working temperature of the planar heat pipe charged with a small amount of non-condensable gas (NCG) can be maintained at a desired level when it is subjected to a sudden heat load or an environmental temperature variation. This advantage makes the variable conductance planar heat pipe (VCPHP) technology appealing to space heat rejection system designers. In the present work, a comprehensive study of the effect of a non-condensable gas on the planar heat pipe performance is conducted. A simple 2D mathematical model was derived from the ideal gas law and flat front assumption. Through this model, the effects of factors such as the NCG inventory, the reservoir size and the sink temperature on the behavior of non-condensable gas as well as on the performance of the planar heat pipe radiator can be analyzed mathematically. To validate this model, a heat rejection experiment was performed. In the experiment, different amounts of nitrogen were carefully measured and injected into the prototype brass planar heat pipe. Temperature profiles were measured and compared with the results calculated from the Steady-State Heat Pipe Operation Model combined with the NCG expansion model. The results indicate that the NCG expansion model is able to predict the location of the vapor-NCG interface as well as the working temperature accurately. In addition, the self-adjustment mechanism of the integral VCPHP is identified to support the applicability of the flat shape Vapor-NCG interface assumption when a non-uniform heat input is applied. (C) 2017 Elsevier Ltd. All rights reserved.