How to get rid of 20 years of smoke

[G. Emerson Zollos]

I am in the process of purchasing a rehab that has been smoked in for the past 20 years. What are some of the most effective ways of removing the smell to achieve showroom quality short of tearing out every piece of drywall.

I should note that we are planning to remove all the carpets and flooring and replace them. We are also planning to paint every room.