At a basic level, Nessus is a vulnerability scanner where as NMAP is used for mapping a network's hosts and the hosts' open ports.
Nmap's main focus is more about detection and auditing, although it's possible to detect vulnerabilities with the scripting engine.
Nmap's scripting engine does a pretty good job of finding vulnerabilities. In general sense, Nessus is a vulnerability scanner and nmap is a port scanner.
Nessus Port scan cannot replace the NMAP port scan