Conditional Forecast

I’m working on the famous Coly Perscott 95 paper.I’ve used the parameter values from the paper and ran the simulation successfully. Now I want to Conditional forecast on variable gdp , actual values taken from US Data. My problem is that I want to conditional forecast yhat(gdphat) with real world gdphat. But I’ve No Clue how to do this.Any Suggestion?
thanks in advance