If I recently quit my job and part of my pay was commission for recruiting clients to the company, am I still entitled to this payment even though I left the company before the payment would have been due?

When it comes to commission- or bonus-based pay, there is no hard-and-fast legal requirement rather, the issue is what were the terms of the agreement even if an only an oral i.e. not written agreement or understanding or demonstrated company policy in regards to such pay. That is, it is legal to have a term or condition that if an employee quits prior to receiving the pay, he or she is not entitled to it. If there was no explicit agreement between you and the company on the subject, look to company past practice what have they done in regards to similar situations or employees in the past? If there is no past precedent, look to industry norms to define the reasonable expectations and hence the likely or reasonable parameters of an agreement in this regard--what do other companies in your space do? 
If there is no firm guidance anywhere about the terms of the agreement, the default or norm would usually be that if you did the work i.e. recruited the clients, you would be paid for it, though it would be reasonable to interpose a slight delay to make sure that the client actually signs ups, pays for services, or doesn't get a chargeback or credit that would reduce commissions. However, if there is an agreement, past company precedent, and/or consistent industry norms that a resigning person in your situation would not get this commission, then that is legal and you most likely are not entitled to it.