In SAGD, it is important to obtain steam chamber conformance along the horizontal wellbore to shorten the ramp-up time and maximize economics. However, reservoir heterogeneity, wellbore undulation, and operation conditions make it hard to achieve this objective. This paper proposes a new method using a condensation temperature model to evaluate the steam chamber conformance of early-period SAGD by interpreting temperature falloff data. The initial temperature distribution of the model is categorized into three zones: a hot-zone of steam temperature, a cold-zone of reservoir temperature, and a transition-zone from steam temperature to reservoir temperature. All three zones are assumed to have circular shapes. This assumption is valid since the model focuses on the early stages of SAGD. The model takes into account the steam condensation on the chamber edge during shut-in, thereby being able to calculate the condensation-front (chamber edge) movement. A semi-analytical approach is developed to solve this model. Sensitivity analysis indicates that the sizes of the steam zone and transition zone, the observing location, and the thermal diffusivity of the formation directly affect the temperature behavior. The synthetic case study shows that the temperature falloffs from the condensation model and from simulation are in good agreement and suggests that the condensation model has the potential to estimate the chamber size during the early stages of SAGD, if calibrated to the field data. Because of the ready-to-use temperature data and the semi-analytic solution, the condensation model proposed in this paper can provide a quick and reliable estimation of the steam chamber size to help the engineers monitor and optimize the chamber development thereafter.