1. A method for determining an estimated value for a sensor which has been removed from a system, comprising: in a computer system: sampling values for performance metrics using external sensors that are coupled to a system and internal sensors that are built into the system, wherein at least one internal sensor is a software sensor; generating an inferential sensing model for the system from the sampled values; and for one or more external sensors that have been removed from the system: sampling values of performance metrics using the internal sensors; and generating estimated values for performance metrics for the one or more removed external sensors, by using the sampled values from the internal sensors as inputs to the inferential sensing model.