SPFile class has CopyTo( ) using which you can copy a document  from one library
to another.
Example:
SPSite site = SPControl.GetContextSite(Context);
SPWeb web  = site.AllWebs["Site_Name"];

SPFileCollection files = web.GetFolder("Source_DocLibrary").Files;

for (int i=0; i<files.Count; i++)
{
if (files[i].CheckInComment == "Complete")
{
files[i].CopyTo("Destination_DocLibrary/" + files[i].Name);
}
}