HackingTheUniverse

This blog entry at Tenable by Paul Asadoorian describes how to most efficiently use both nmap and nessus together without duplicating effort.

Using Nmap Results With Nessus Batch Scanning – [tenablesecurity.com]

Conclusion
If you have previously scanned your network with Nmap and saved the results in grepable Nmap output, you can save the step of having Nessus rescan your network by importing the results into Nessus and incorporating them into your scan. The Nessus command line client (“nessus”) can import results and scan polices from a .nessus file. Large organizations who have dedicated port scanning monitoring efforts should also consider deploying the Passive Vulnerability Scanner with the Security Center. This allows continuous monitoring of the network for change in real time and automatically combines passively discovered information with results from Nessus active and credentialed network scans.