1. A method for estimating the value of a virtual sensor, comprising: sampling values for performance metrics using external sensors that are coupled to a system and internal sensors that are built into the system; generating an inferential sensing model for the system from the sampled values; removing the external sensors from the system; and during operation, sampling values of performance metrics using the internal sensors, and then using the inferential sensing model and the sampled values from the internal sensors to estimate the values of performance metrics for the removed external sensors.