NUnit Test Launcher bundles support for .NET code coverage using NCover, PartCover and dotCover coverage engines.
Details on configuring code coverage can be found on the corresponding pages:
Coverage configuration via TeamCity UI is only supported for the cases when the the tests are run using TeamCity-managed test launchers. Specifically these covers:
NAnt runner: <nunit2> NAnt task
MSBuild runner: <NUnit> and <NUnitTeamCity> MSBuild tasks
Any runner: TeamCity NUnit Test Launcher
If you use a test framework other than NUnit, you can configure coverage analysis manually using the JetBrains dotCover console runner and TeamCity service messages as described in Manually Configuring Reporting Coverage.