Thermodynamic steam-trap control, or subcool control, in a typical steam-assisted gravity-drainage (SAGD) production is essential to the stability and longevity of the operation. It is achieved commonly through the control of fluid production. The goal of such control is to maintain a steady and healthy liquid production without allowing steam from the injector to bypass to the producer. Therefore, it is effectively a control of the liquid level above the producer. Unfortunately, it is not practical to monitor this liquid level. A rule-of-thumb subcool-per-metre estimation of 10 degrees C/m of liquid level is popular in the industry; however it does not prove to hold in many situations. This paper presents a study of the dynamics of SAGD-production control with a resulting algebraic equation that relates subcool, fluid productivity, and wellbore drawdown to the liquid level above a producer. The main conclusions of this study include There is no minimum subcool value for a pure-gravity-drainage scenario; however, as the wellbore drawdown is considered, there is a minimum subcool value in order to maintain the stability of fluid flow. For a given productivity, the liquid level increases as subcool increases or as wellbore drawdown decreases. For each given set of operating parameters, there exists a critical productivity below which SAGD operation would halt. Before the steam chamber reaches the top of the reservoir, the fluid productivity is limited by the vertical distance between the injector and the producer; the larger the distance, the higher the fluid-production rate can be. A verification of this analysis was conducted by a series of numerical reservoir simulations. Although limited to two dimensions, we expect that this analysis captures the main physics amid the dynamic complexity of SAGD-production control. The resulting algebraic equation can be used for better understanding of the dynamics of subcool control and for determining operation strategies.