qgis - Producing singlepart featureclass with unique IDs for each spatially non-connected polygon?

What is the best method to convert a singlepart shapefile (i.e. one record that contains several non-connected polygons) into a mutlipart shapefile?

For clarification the original question asked how to produce a singlepart featureclass with unique IDs for each spatially non-connected polygons.

The explode method in Advanced Editing or Multipart to Singlepart would give this result.

Answer

If you would like to convert a polygon feature class with a single record to a polygon feature class with a record for each polygon, use Multipart to singlepart (Data Management) aka "explode".

To convert single-part polygons to multi-part polygons, you can use the dissolve tool in ArcGIS. Make sure to dissolve by OBJECTID and select "Create multipart features". ArcGIS help also has a section on creating a multipart feature.